The below should do the trick. You'll need the latest release (2.4.0) for this to work however.

Type: Redirect
Trigger: onStartSaveUserRegistration
User: Automatic
Access: Everybody
Condition: [post_email] Not Equal To [post_cb_confirmedemail]
URL: goback

Believe this should give you what you're wanting, but you may need to adjust the URL or trigger to give a better experience such as maybe needing the on before registration trigger for example.

Ah, right forgot that registration goes through regardless of user abort. I think you'll also need a duplicate of the redirect action except it being a Code action with the PHP code "exit()" or "die()" so it stops PHP from continuing with the registration. Example as follows.

Type: Code
Trigger: onStartSaveUserRegistration
User: Automatic
Access: Everybody
Condition: [post_email] Not Equal To [post_cb_confirmedemail]
Method: PHP (create_function)
Code: die();
Return: Silent

Ensure the code action is ordered after the redirect action.